1. Field of the Invention
This invention relates to a pigment ink suitable for ink jet recording as well, which makes use of a pigment as a coloring material, and also relates to an ink jet recording method, an ink cartridge, a recording unit and an ink jet recording apparatus which make use of the pigment ink.
2. Description of the Related Art
In inks for ink jet recording, pigment inks, which contain pigments as coloring materials, are widely used in recent years not only in small-sized printers used for office work and domestic purposes but also in large-sized printers used for the purpose of printing posters and advertisements. In the case of printing with large-sized printers, fairly large recording mediums of, e.g., A0 size and A1 size are often used, and it is common for them to be rolled up into cylindrical shape when recording mediums on which images have been formed are carried. Hence, problems as stated below may arise.
When a recording medium is rolled up, images may be rubbed at edged portions such as corners of the recording medium. On such an occasion, images formed using pigment inks are scratched and coloring materials are scraped off. Such a problem may fairly frequently arise. Problems like this may also arise in other circumstances. For example, there is a case in which, when images formed using pigment inks are posted outdoors as posters, the images are strongly scratched with something edged such as finger nails. In such a case as well, as in the foregoing, such a problem that coloring materials are scraped off fairly frequently arises. Thus, in the case where pigment inks are employed as the inks to be used in the large-sized printers, it is an important subject to achieve scratch resistance at a strikingly higher level than ever, which is high enough for coloring materials of images not to be scraped off even when something sharp touches the images.
To cope with this matter, since the scratch resistance of the images formed using pigment inks has hitherto been a subject, various methods are proposed in order to resolve such a subject. However, most of studies involved in such proposals have been aimed at the small-sized printers used for office work and domestic purposes. Accordingly, what is the best that can be given by the use of conventional techniques alone is to achieve scratch resistance at such a level that images are not scratched upon touching the images. That is, even now, the subject of the scratch resistance of the images when the image is touched by something edged as mentioned above still has been unable to be resolved.
Of course, some proposals have been made for achieving scratch resistance of images at a higher level so that images can be provided which can satisfy the scratch resistance even under such circumstances as noted above. For example, there is a proposal concerned with an improvement in scratch resistance of images which is achieved by applying a specific compound onto images formed using an ink jet recording ink to provide the images with high slipperiness (see Japanese Patent Applications Laid-open No. 2000-108495 and No. 2004-284362; hereinafter “Patent Documents 1 and 2”, respectively). There is another proposal concerned with an improvement in scratch resistance and abrasion resistance of images which is achieved by using an ink containing a specific polymer and a specific polyether modified organosiloxane (Japanese Patent Application Laid-open No. 2003-192964; hereinafter “Patent Document 3”).
However, studies made by the present inventors have revealed that, where images are formed using only ink jet recording inks, the images formed do not satisfy the above scratch resistance at the strikingly high level at which the present invention is aimed, even though any of the conventional techniques as noted above is used. That is, in the conventional techniques as noted above, satisfactory images may be obtained as long as the scratch resistance is at such a level that images are not scratched when touched gently with finger nails, but scratch resistance of images at a level higher than that has been unable to be achieved. Specifically, a phenomenon occurs in which coloring materials are scraped off when images are scratched with something edged such as finger nails under application of such strong pressure as to scratch a recording medium at its non-recorded areas.
For example, the objective of the present invention may be achievable if, after images have been formed, the images are protected sufficiently by using a method in which images are protected with the so-called solid material, e.g., by spraying or covering them with a lubricating substance or a slip compound, as disclosed in Patent Documents 1 and 2. The objective of the present invention may be achieved as in the invention disclosed in Patent Documents 1 and 2, also when a laminate system or the like is used in which images are covered with a film. However, as a result of the investigation of the present inventors, as disclosed in Patent Document 1, where an ink containing a lubricating substance is used or an aqueous solution containing a lubricating substance is applied onto images, the lubricating substance penetrates into a recording medium together with an aqueous medium. As a result, the effect of improving scratch resistance that is to be brought about by applying such liquids onto images has been found to be hardly obtained. That is, with the method in which the ink containing such a substance as to merely improve scratch resistance is used or the aqueous solution containing said substance is applied onto images, the scratch resistance at which the present invention is aimed cannot be achieved. When the laminate system is employed, there is a problem that cost becomes high because a laminating treatment is conducted by another machine other than recording apparatus.
The method disclosed in Patent Document 3 also can not achieve images having the scratch resistance at a strikingly high level at which the present invention is aimed, as in the case of Patent Documents 1 and 2. More specifically, according to Patent Document 3, it is disclosed that an ink containing a specific polymer and a specific polyether-modified organosiloxane is used in order to achieve both abrasion resistance and scratch resistance. However, in the examples of the Patent Document 3, upon making a comparison between Example 1 in which the specific polyether-modified organosiloxane is contained and Comparative Example 2 in which the specific polyether-modified organosiloxane is not contained, the abrasion resistances of them are equal to each other. That is, in the invention disclosed in Patent Document 3, the polyether-modified organosiloxane in the ink penetrates into a recording medium together with an aqueous medium after the ink has been applied onto the recording medium, and hence it is indicated that the organosiloxane is not involved in the effect of improving the abrasion resistance. In other words, it can be said that the invention disclosed in Patent Document 3 suggests that the improvement of abrasion resistance depends only on the polymer.
On the other hand, in regard to the scratch resistance of images, the effect attributable to the polyether-modified organosiloxane is somewhat seen. However, taking into account the fact that the scratch resistance is abruptly improved when a certain time has lapsed after the images have been formed, it can be said that the effect of improving the scratch resistance also depends primarily on the polymer. As a result of studies made by the present inventors, in the case of only the performance of such a water-soluble polymer as used in ink jet recording inks, the binding force between a recording medium, a polymer and a pigment depends only on the strength of the polymer. Hence, it was found that however much the polymer is contained in the ink, only scratch resistance is achievable which is at such a level that the images on the recording medium are not scratched when touched gently with finger nails. Especially where the content of the pigment solid matter in the ink is 1.2% by mass or less based on the total mass of the ink, the solid matter that functions as filler is so small that the high scratch resistance at which the present invention is aimed cannot be achieved by only the polymer. That is, also in the invention disclosed in Patent Document 3, in which the scratch resistance is improved relying only on the performance of the polymer, the scratch resistance is at a level classified as conventional one, and does not at all reach the level of scratch resistance at which the present invention is aimed.
In addition, the ink disclosed in Patent Document 3 can be said to be not so suitable for an ink jet recording ink. The polymer used in the invention described in Patent Document 3, which has an extremely low acid value, is very low in solubility in an aqueous medium. Hence, when the ink is prepared, such a polymer must be dissolved at a fairly high temperature of 100° C. or more. This is impractical for an ink used in common ink jet recording methods. Besides, as in the ink disclosed in Patent Document 3, when applying to an ink jet method an ink the film forming properties of which is rapidly improved with a lapse of time after being ejected, the ink may dry in nozzles to cause faulty ejection when left standing in the state that the ink is not ejected for a certain time. Further, where such an ink is ejected by a thermal-type ink jet recording method, stable ejection property is difficult to maintain because the polymer has too low an acid value. Taking these into account, it can be said to be difficult to utilize the technique disclosed in Patent Document 3 to improve the scratch resistance of images.